Most of the information is here:
https://steemit.com/faq.html#What_is_Steemit_com

But basically, you get rewarded in tokens for either writing a popular post (Author rewards) or by voting for a post within a certain time frame that then becomes popular (Curation rewards).

Your votes carry more weight, and give more rewards proportionally, the more STEEM POWER your account has and also reduce in weight if you have voted a lot recently. You can choose to split payouts by STEEM and STEEM POWER since STEEM is more liquid but STEEM POWER helps you build up more rewards in future.

Hopefully that explains the basics, there is more info in the above link.